Good idea, I probably write a blog post on this later. If you take a look at [1], I explain the difference between N:1 and M:N mappings. StateThreads uses a N:1 mapping which means you can multiplex many fibers over a single thread and to take advantage of multi-processors you can have M processes that do N:1 mapping, which is a common practice (libmill, libdill, ...). But with uThreads you can multiplex many fibers over many kernel threads.
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[1]https://github.com/samanbarghi/uThreads#what-are-uthreads